1. Field of the Invention
The present invention relates generally to a method for reducing byproducts emissions from combustion reactions, and, more particularly, to a method for reducing flue gas acidity in combustion furnaces.
2. Description of the Prior Art
Acidity Decrease
The flue gas of power generation plants has long been recognized as a source of atmospheric pollution. In the combustion of fossil fuels, some of the naturally present elements are oxidized to form acids, such as SO3, NOx, HCl, HF, and the like. These acids, especially SO3, can become a problem if their concentrations exceed certain thresholds. For example, as the SO3 concentration increases, the acid dewpoint temperature of the flue gas increases. If the temperature of the flue gas is less than the acid dewpoint temperature of the flue gas, the SO3 in the gas will condense and react with water to form H2SO4, causing corrosion problems inside the furnace. Also, flue gases exiting a furnace cool immediately and SO3 and other acids in the gas condense, creating localized acid rain, which is the condensation and precipitation of SO3 and other acids onto the surrounding land with subsequent corrosion. Excessive SO3 will condense into small droplets, creating a visible plume as it exits the furnace, which becomes an esthetic and local political problem. If NH3-like compounds are present in the flue gas, they can react with SO3 to form ammonium bisulfate (NH3HSO4) which then fouls the air heater.
Thus, a need exists to decrease the acid dewpoint temperature of the flue gases such that the acid dewpoint temperature is lower than the flue-gas temperature in the coolest parts of the furnace, such as the ducts and stack. A further need exists to lower the acid content of the flue gases such that the localized acid rain and other problems associated with high-acid flue gas are minimized.
SO3 Increase
The particulate matter carried in the flue gas can be removed by electrostatic precipitators that cause the individual particles to accept an electrical charge and then use that charge to attract them to collector plates for disposal. The efficiency of such electrostatic precipitators is dependent upon the ability of the individual particles to take a charge, that is, the resistivity of the particles. It has been found that the presence of SO3 in the flue gas effectively reduces the resistivity of the particles, making them easier to charge electrostatically.
In the combustion of coal, some of the naturally present sulfur is converted to SO3. On the other hand, the effectiveness of SO3 in reducing the resistivity of the particulate matter in the flue gas depends upon the concentration of the SO3, with about 15 to 20 parts per million (ppm) giving optimal results. Therefore, precipitator efficiency is affected by the ability to adjust the amount of SO3 in the flue gas, regardless of the sulfur content of the coal being burned, to provide an overall SO3 concentration in the optimal range.
SO3 is also produced in SCR (catalyst) installations by the oxidation of SO2 and often exceeds the optimal 15 to 20 ppm optimal concentrations. The catalyst blends typically used in the SCR to reduce NOx to N2 (in the presence of ammonia) also oxidize SO2 to SO3. The rate of this reaction is strongly temperature dependent and, at higher temperatures, can convert more than 1 percent of SO2 to SO3. High sulfur U.S. coal generates anywhere from 2,000 to 3,000 ppm of SO2 in the boiler, and therefore can result in 20 to 30 ppm of SO3 out of the SCR. The problem is that as much as 50 percent, or 10 to 15 ppm, of the SO3 coming out of the SCR will make it past the scrubber and out of the stack. At about 8 to 10 ppm, depending upon the particulate concentration, SO3 becomes visible as a blue plume.
Furthermore, SO3 can also be produced catalytically on other boiler surfaces through interaction with elements/chemicals such as Vanadium.
Therefore, because any SO3 formed prior to the SCR adds to the effluent SO3, reducing the SO3 formed prior to the SCR is important for reducing the effluent SO3 and permits the use of SCR for the reduction of NOx for gases without generating excessive amounts of SO3.
SO3 Control
If the SO3 concentration is too low, the precipitator will operate at less than optimal efficiency. On the other hand, if the SO3 concentration is too high, the flue gas becomes highly acidic, creating a “blue plume” and contributing to acid rain. In addition, acidic flue gases contribute to corrosion of the pipes carrying the flue gas, and, when combined with NH3-type chemicals, can clog the air heater.
Furthermore, an SCR is often only intended to be used for six months per year (during the summer ozone control season), and are bypassed during the winter. This creates seasonal variability in the SO3 concentrations at the precipitator, in the duct work, and out of the exhaust stack.
It is therefore desirable to control the concentrations of SO3 in the flue gas depending upon whether the SCR is in use or not. SO3 concentrations approaching 40 ppm produce severe adverse local acid problems that are not necessarily regulated, but create local political problems for the facility. The U.S. EPA has indicated that future regulations on SO3 emissions are to be expected.
It is desirable, therefore, to have an SO3 flue gas system that is capable of adjusting the concentrations of SO3 in a flue gas with or without an SCR installed to maintain the SO3 concentration at an optimal level for increased ESP performance, without increased localized SO3 emissions.
Staging
Combustion staging is the process of burning a fuel, i.e., coal, in two or more stages. A fuel-rich stage, or simply, rich stage, is one in which not enough air is available to fully burn the fuel. A fuel-lean stage is one in which there is sufficient or extra air to fully burn the fuel. Staging is used in the prior art to reduce NOx by a) reducing peak temperatures (thermal NOx) and b) providing a reducing environment (NOx reduction). Macro-staging is the dividing of whole sections of a furnace into rich and lean stages and is accomplished through the use of such techniques as Over-Fired Air (OFA). Micro-staging is the creation of proximal microenvironments with functionally different characteristics, such as reduction potential, temperature, and the like. Micro-staging in a furnace can be achieved, for example, in the first stage of the furnace through the use of Low-NOx burners with adjustment of spin-vane settings and registers. Increased staging increases the residence time in a reducing atmosphere and increases the effect of the reducing atmosphere.
Prior art has used micro-staging to reduce NOx emissions in combustion furnaces. Low-NOx burners (LNB) stage by delivering high-fuel-content primary air into the furnace that mixes with secondary air flowing through one or more secondary air registers. LNB primarily use micro-staging. The flow through a LNB is designed such that the volatile components of the coal mix with the available near-field air at a stoichiometric ratio near unity (1.0), thus anchoring the flame. The net combustion in the central core near the burners is overall fuel rich and does not produce much thermal NOx, as the temperatures are low. The coal is eventually consumed over the depth of the furnace as more and more air slowly mixes into the central core. The majority of the NOx created in this region is from the fuel-bound nitrogen reacting to NO through the intermediate HCN. The rate at which the outer secondary air mixes into the core flow is set by the dampers and the spin vanes, as well as the spin vane in the coal pipe. LNB systems decrease NOx by staging since there is a continuous mixing of the rich products of combustion and secondary air throughout the combustion zone. Staging is increased by decreasing the mixing rate between the rich core flow and the outer secondary air flow.
Prior art has used macro-staging to reduce emissions in combustion furnaces. Macro-staging consists of highly mixed fuel and air in the lower furnace, mixed to a stoichiometric ratio below unity for a large part of the flow. Excess oxygen is ultimately required to assure that all of the fuel has burned and to reduce explosion risks. In a macro-staged furnace, excess air is introduced downstream of the burners. Increased staging is achieved by increasing the residence time, temperature, or reducing quality of the combustion products in the absence of oxygen.
Prior art used both micro-staging (LNB) and macro-staging (OFA) to reduce NOx emissions in combustion furnaces. In the case of both micro-staging and macro-staging, components of each of the above are used and adjusted to achieve NOx emissions reduction.
Staging has nowhere been taught in the prior art for flue gas acidity reduction, acid dewpoint temperature control or SO3 concentration control in combustion gases.